1. Field of the Invention
The present invention relates generally to semiconductor light-emitting devices and, more particularly, to light emitting devices which employ an efficient reflective mirror to minimize optical loss by substrate absorption.
2. Discussion of the Prior Art
High efficiency visible light emitting devices (LED) are highly desirable for such applications as large area displays, traffic signal lighting and automotive lighting. Even though the internal quantum efficiency of a visible LED may be very high, its luminescence performance is typically limited by the light extraction efficiency. Among other factors such as the internal reflection loss at the chip surface and the blocking of light by the contact electrode, the light absorption by the substrate represents a major optical loss of the light emission. Conventional approaches to achieving reduction of substrate absorption loss include the use of a semiconductor Bragg reflector (BR) and the use of transparent-substrate LED.
S. Murasato et al have disclosed, in U.S. Pat. No. 5,744,829, an AlGaInP LED on GaAs substrate containing a BR using multiple layers of AlGaAs between the substrate and the double hetero-junction (DH) light-emitting structure. For example, a BR comprising ten to twelve pairs of [Al0.4Ga0.6Asxe2x80x94Al0.95Ga0.05As] was proposed to provide a reflectivity of 90% or more at an emission wavelength of 570 nm. Such a high reflectivity is only obtained if assuming a fixed refractive index of 3.7 for Al0.4Ga0.6As and 3.146 for Al0.95Ga0.05As with no dispersion and non-absorbing. However, the index dispersion and absorption of the lower-index layer are important in affecting the reflectance of the BR. As described by D. E. Aspnes et al in J. Appl. Phys. 60 (1986) pp. 754-767, the real (n) and imaginary (k) part of the complex index of refraction of the AlGaAs layers have a finite value and varies with the emission wavelength. A more realistic estimate of the reflectance of the proposed BR is less than 60%. The calculated reflectance spectrum of the [Al0.4Ga0.6Asxe2x80x94Al0.95Ga0.05As] BR is shown in FIG. 1a using the AlInP lower confining layer as the incident medium. The reflectance of the BR drops rapidly for light emitting at an angle greater than 22 degree. The angular dependence of the 570-nm reflectance of the BR is shown in FIG. 1b. 
H. Sugawara et al have described a high reflectivity of 70% using a 10-pair [AlInPxe2x80x94GaAs] BR or a 20-pair [AlInPxe2x80x94(Al0.5Ga0.5)0.5In0.5P] BR in Jpn. J. Appl. Phys. Vol.33 (1994) Pt.1, pp. 6195-6198. The reflectance bandwidth of the [AlInPxe2x80x94GaAs] BR was found to be greater than that for the [AlInPxe2x80x94AlGaInP] BR. The authors have assumed a fixed value of the refractive index and absorption coefficient of each layer in their calculations. However, a strong dispersion of the refractive index (n and k) of AlGaInP layers especially near the band edge has been determined using the ellipsometry method as reported by M. Moser et al in Appl. Phys. Lett. Vol.64 (1994) pp.235-237 and using the reflectance method as reported by Y. Kaneko et al in J. Appl. Phys. Vol.76 (1994) pp.1809-1818. Taking into account the dispersion relationship of the refractive index for each layer, the maximum reflectance at a wavelength of 570 nm is found to be below 35% for the [AlInPxe2x80x94GaAs]20 BR and [AlInPxe2x80x94(Al0.5Ga0.5)0.5In0.5P]20 BR. This value is only half of that published by H. Sugawara et al. The calculated reflectance spectrum and the angular variation of the 570-nm reflectance are shown in FIG. 2a and FIG. 2b for the [AlInPxe2x80x94GaAs]20 BR, and in FIG. 3a and FIG. 3b for the [AlInPxe2x80x94(Al0.5Ga0.5)0.5In0.5P]20BR, respectively.
H. Sugawara et al have also described a wide-band, high reflectivity BR using hybrid [AlInPxe2x80x94GaAs]xe2x80x94[AlInPxe2x80x94AlGaInP] multiple layers. The reflectivity of the hybrid-type BR is 70% at an emission wavelength of 570 nm if assuming a non-dispersive n and k for each layer. However, actual reflectance of the hybrid-type BR is only 26% due to the dispersion of the refractive index for each layer. The calculated reflectance spectrum and the angular variation of the 570-nm reflectance of the [AlInPxe2x80x94GaAs]10-[AlInPxe2x80x94AlGaInP]10 hybrid BR are shown in FIG. 4a and FIG. 4b, respectively. The low reflecting power and high angular sensitivity of the prior art BR design has restricted the overall light-extraction efficiency of the LED.
The fractional bandwidth of a quarter-wave stack BR is given by
xcex94"sgr"/"sgr"o=4/xcfx80arc sin (nH/nLxe2x88x921)/(nH/nL+1)
where "sgr"o=1/xcexo and xcexo is the wavelength at which the layer thickness are quarter-waves. The nH/nL represent the refractive-index ratio of the high- and low-index layer, respectively, as described by O. S. Heavens et al in Applied Optics, Vol.5 (1966) pp.373-376. FIG. 5 shows that the fractional bandwidth increases as the refractive-index ratio increases. The prior art semiconductor BR has a small refractive-index ratio in the visible region of the spectrum and a narrow reflectance band. For example, using index values of 3.289, 3.225, 3.998 for AlInP, AlAs and GaAs layers, respectively, one obtains an index ratio of 1.2 for the [GaAsxe2x80x94AlAs] and [GaAsxe2x80x94AlInP] quarter-wave stacks at a wavelength of 570 nm. This corresponds to a fractional bandwidth of only 0.14 for the prior art BR. A larger ratio is clearly preferable for broad bandwidth applications. However, it is impractical to find semiconductor materials with an index ratio of greater than 2 while electrically conductive and lattice-matched to the substrate.
The small refractive index differential in the prior art BR design has practical drawbacks in the LED production. It requires a large number of pairs and long crystal growth cycle to obtain high reflectance. The narrow reflection bandwidth of the BR means a small fabrication tolerance and demands rigorous control of epitaxial growth process. A broad bandwidth BR is also desirable due to the broad spontaneous emission spectrum of the LED.
The aforementioned deficiencies are addressed, and an advance is made in the art, by a semiconductor light-emitting device which employs an efficient reflector to minimize optical loss due to substrate absorption. The reflector comprises structured quarter-wave stacks deposited on a patterned substrate that is configured to allow for current injection around the discrete reflector stacks. The reflector is further characterized by a high refractive-index ratio (nH/nL) suitable for broad-bandwidth high reflectance application of the light-emitting device.
A device constructed in accordance with an illustrative embodiment of the invention is obtained by forming Bragg reflector (BR) stack portions on a patterned substrate using conventional epitaxial growth methods. Illustratively, a surface of the textured substrate defines a channel structure wherein surface portions of different elevation are realized. The specific geometry of the textured substrate is selected so as to facilitate the discrete deposition of the BR layers on top of the hills and in the valleys. The region between the isolated BR stacks is reserved for paths of current injection.
After the growth, the wafers fabricated in accordance with the illustrative method are provided with a surface groove member to expose high aluminum-content AlGaAs layers in the BR. The wafers are then transferred to a furnace and steam oxidized to laterally convert the high aluminum-content AlGaAs layers into AlOx. The as deposited GaAsxe2x80x94AlAs BR is thus converted into GaAs-AlOx BR as a result of the selective oxidation of the AlGaAs layer. The new GaAs-AlOx BR has a high index ratio of 2.26 due to the very low refractive index of 1.77 for AlOx. Highly reflective, broad bandwidth BR is thus materialized using the new structured BR in the present invention. The reflector stack in the present invention has a broadband, high spectral and angular reflectivity suitable for the fabrication of high efficiency bright LED""s.
The present invention will be best described in detail with reference to the figures listed and is described below.